Recently it has been demonstrated that covalent bonded ceramics, such as silicon carbides (SiC, SiOC), silicon nitrides (Si3N4, SiCN), aluminum nitride (AlN) and hafnium carbide (HfC) can be synthesized by thermal decomposition of suitable polymeric precursors, and are referred to as polymer derived ceramics (PDCs).
Traditionally, bulk PDC objects can only be made through a powder-compact-pyrolysis route, which sacrifices many of the advantages of the technique.
Edwin Kroke et al in “Silazane Derived Ceramics and Related Materials” Materials Science and Engineering, 26 (2000) pages 97-199, provide a state of the art review of the synthesis, process and properties of non-oxide silicon-based ceramic materials derived from silazane and polysilazane precursor compounds. Kroke et al acknowledge the evolution of elaborate ways to utilize especially designed and synthesized silazanes, as well as commercial Si-based polymers. The demonstrated processing of silazanes to ceramic materials include the classical approach by liquid phase sintering of silazane-derived amorphous Si/C/N powders with additives and the non-classical approach which involves the direct transformation of molecular and polymeric silazanes to silicon nitride/carbide-based ceramics. Kroke et al. concluded that further development of the silazane derived silicon-based non-oxide ceramics is needed. It is specifically mentioned that formation and amount of volatile side products during the polymer-to-ceramic transformation has to be controlled and minimized.
In the quest for an ideal dense silicon-based bulk ceramic material, with desired properties, ease of manufacture, low costs and availability of raw materials, several scientific journal articles report on synthesis techniques. For example, R. Riedel et al in “Synthesis of Dense Silicon-Based Ceramics at Low Temperatures” Nature 335 20 February 1992, pages 714-717, reports the direct transformation of a metallorganic precursor into non-oxide silicon-based ceramics with relative densities of up to 93% in a process for making ceramic components and matrix composites at unusually low temperatures (1000° C.) using an infusible polymethylsilazane powder and without the addition of sintering aids.
R. Riedel et al. in “Polymer-Derived Si-Based Bulk Ceramics, Part I: Preparation, Processing and Properties” Journal of the European Ceramic Society 15 (1995) 703-715 describes the synthesis and processing of dense silicon-based bulk ceramic materials from the thermal decomposition of preceramic organosilicon polymers such as polysilazanes and polysilanes via three different routes using infusible polysilazane powders or powder blends as the starting materials.
C. Konetschny et al. in “Dense Silicon Carbonitride Ceramics by Pyrolysis of Cross-linked and Warm Pressed Polysilazane Powders” Journal of the European Ceramic Society 19 (1999) 2789-2796 reports on the pyrolysis and densifactiion behavior of cross-linked poly(hydridomethylsilazane) powders.
Peter Greil in “Polymer Derived Engineering Ceramics” Advanced Engineering Materials 2000 (2) No. 6 339-348 reports that bulk components can be produced using preceramic polymer binders that yield a higher green density especially with powders of low packing density. Filler powder surface may also be used to control shaping and curing behavior.
Li-Anne Liew et al. in “Fabrication of SiCN Ceramic MEMS Using Injectable Polymer-Precursor Technique” Sensors and Actuators A 89 (2001) 64-70 report on cost-effective technology for the fabrication of high-temperature MEMS based on injectable polymer-derived ceramics where liquid-phase polymers are cast into molds and converted into monolithic, fully dense ceramics by thermal decomposition.
Sandeep R. Shah et al in “Mechanical Properties of a Fully Dense Polymer Derived Ceramic Made by a Novel Pressure Casting Process” in Acta Materialia 50 (2002) 4093-4103 describes a two-step process for obtaining fully dense samples of silicon carbonitride (SiCN) from polymers including a first step of cross-linking the liquid organic precursor under pressure, followed by a second step of controlled pyrolysis. Net shape processing is possible by casting the liquid into a mold before polymerization.
Rahul Harshe et al in “Amorphous Si(Al)OC Ceramic from Polysiloxanes: Bulk Ceramic Processing, Crystallization Behavior and Applications” in Journal of the European Ceramic Society 24 (2004) 3471-3482 discusses bulk Si—Al—O—C ceramics produced by pyrolysis of commercial poly(methylsilsesquioxane) precursors. Prior to the pyrolysis the precursors were cross-linked with a catalyst, or modified by the sol-gel-technique with an Al-containing alkoxide compound, namely alumatrane. Modification of polysiloxane by alumatrane forms dense, crack-free SiAlOC ceramics with improved high temperature stability as compared to Al-free SiOC compositions.
None of the prior art references use a simple, inexpensive, process for forming dense, crack-free bulk ceramic structures. The novel process and products of the present invention meet a commercial need for the production of bulk ceramic parts in a non-labor and energy intensive process, providing a green body that is tough and can be easily machined.
As compared to the traditional powder metallurgy based ceramic processing, the technique of the present invention has many advantages, including high purity, low temperature processing and no sintering aids.